The prototype of an online Content-Based Art Retrieval (C-BAR) system is introduced that provides entrance to the digitized collection of the National Gallery of the Netherlands (the Rijksmuseum). The current online system of the Rijksmuseum is text-based and requires expert knowledge concerning the work searched for, else it fails in retrieving it. C-BAR extends this system with querying by an example image, which can be provided to the system or can be selected through browsing the collection. The global color distribution of the example image are extracted and compared with those of the images in the collection. Hence, based on either text or content-based features, the collection can be queried. Moreover, the matching process of C-BAR can be inspected. With the latter feature, C-BAR not only integrates the means to inspect collections by both experts and laypersons in one system but also provides the means to let the user to understand its working. These characteristics make C-BAR a unique system to access, enhance, and retrieve the knowledge available in digitized art collections.